A split domain configuration allows customers to host a select number of mailboxes on the Zimbra server while the bulk of users are hosted by a third-party service or an in-house server. Split domains are typically used to save on email hosting costs by having most users access their mail at a simple POP or IMAP provider, while certain users that require more robust email capabilities can be set up with Zimbra mailboxes.

Note: Because a split domain configuration utilizes multiple email providers and locales, and much of the configuration for the split domain exists outside our network, support for split domain configurations is limited.


Split Domain Delivery Path


When a company uses a split domain configuration, they are effectively operating two separate email providers. The first provider, which contains the bulk of their users, is typically either a basic POP provider or an in-house server. Email that is addressed to the company will route to this provider first. Any email that is configured at the Zimbra provider will then be forwarded from the first provider to the Zimbra provider using a sub-domain.


If a company uses 'mydomain.com' as their domain, they will need to configure two sets of Mail Exchanger (MX) DNS records for that domain:


• The MX records for 'mydomain.com' point to the mail servers for the first provider (POP/In-House)

• The MX records for 'zimbra.mydomain.com' point to the mail servers for the Zimbra provider


Once the MX records have been properly pointed, forwarding will need to be configured at the first provider for each user that will be set up on the Zimbra host. For example:


• If user johnsmith@mydomain.com has a Zimbra mailbox, their mailbox at the first provider will be configured to forward mail to johnsmith@zimbra.mydomain.com.


So the mail flow for a Zimbra user is as follows:


Sender > johnsmith@mydomain.com > POP/In-house server > forward to johnsmith@zimbra.mydomain.com > Zimbra server.


To Request a Split Domain Configuration


Requests for a Split Domain can be made by sending a ticket request to support if you are unable to send it using the control panel. In this request, we will need the following information:


• The host-name of the remote mail server (or first provider).

• Verification that the MX records for zimbra.yourdomain.com have been configured and are pointing to '‘domain.com.mf.rp.securembox.com’ (without quotes)


You will also need to ensure that the desired Zimbra users have been created in your Zimbra Admin Console, and that forwards have been set up at the first provider for those users to route to user@zimbra.yourdomain.com.


If you use SPF for your primary domain, we recommend adding our outbound host-name to your SPF record to ensure Zimbra users will be able to deliver mail normally. The host-name is as follows:


yourdomain.com IN TXT "v=spf1 include:_spf.securembox.com ~all"